This invention relates to the separation and purification of precious metals which consist, in the context of this specification, of the platinum group metals and gold. More particularly the invention is concerned with the separation and/or purification of what are commonly termed secondary platinum group metals or by-metals, which consist of rhodium, ruthenium, iridium and osmium.
In all processes within applicants' knowledge a precious metal concentrate is initially split into two groups, one of which consists basically of platinum, palladium and gold, and the other of which comprises the by-metals. In one known process the by-metals are alloyed with lead to separate a substantial amount of base metals therefrom initially. ("Base metals" in the context of this specification include all metals apart from precious metals and silver). Lead is then dissolved using dilute nitric acid and platinum, palladium and gold are dissolved by means of aqua regia.
The resultant reside constitutes a feed to a by-metal separations and purification stage of the whole process. As an example of the type of concentrate which may be obtained from the above process steps, the following is given by way of illustration with the relative proportions of the metals being given in weight percentages:
______________________________________ ELEMENT PERCENT ______________________________________ Platinum (Pt) 1 - 5 Palladium (Pd) 1 - 3 Gold (Au) 0,1 - 0,5 Rhodium (Rh) 5 - 15 Ruthenium (Ru) 30 - 50 Iridium (Ir) 4 - 10 Osmium (Os) 0 - 5 Silver (Ag) 2 - 5 Copper (Cu) 0 - 4 Nickel (Ni) 0 - 4 Iron (Fe) 0 - 1 Lead (Pb) 20 - 50 Tellurium (Te) 0 - 1 Bismuth (Bi) 0 - 0,5 Arsenic (As) 0 - 0,5 ______________________________________
Such a by-metal concentrate may be fused with potassium bisulphate (KHSO.sub.4) which renders the rhodium water soluble by converting it to the sulphate, Rh.sub.2 (SO.sub.4).sub.3.
After water treatment to remove rhodium, the residue is separated from the dissolved rhodium and subjected to a sodium peroxide (Na.sub.2 O.sub.2) fusion which converts the ruthenium and osmium to water soluble sodium salts of the oxo-anions of the two metals (e.g. RuO.sub.4.sup.2.sup.- and OsO.sub.4.sup.2.sup.- respectively) and iridium to an acid soluble hydrous oxide probably IrO.sub.2.nH.sub.2 O. The ruthenium and osmium are then separated from the iridium by treating the sodium peroxide melt with water followed by filtration while the iridium is dissolved by treating the resultant residue with hydrochloric acid.
The metals thus separated from one another are then treated for further purification. The ruthenium and osmium are normally purified by a collective chlorine distillation, followed by a nitric acid distillation for osmium. The rhodium is treated for the removal of impurities such as palladium, tellurium and other base metals which are also rendered soluble by the KHSO.sub.4 fusion. The iridium has to be separated from large quantities of lead and other impurities present in the concentrate which have been rendered soluble by the sodium peroxide (Na.sub.2 O.sub.2) fusion.
The above process is often carried out on what applicant considers to be unecessarily large quantities of concentrates thereby using correspondingly large quantities of costly reagents. Also, the impurities, in particular tellurium are sometimes difficult to remove.